Generally, a laser diode (LD) provides relatively good monochromaticity and directionality, relatively smaller sizes and/or dimensions, and high usage of optical power. However, the environment has a significant impact on the output optical power from conventional laser diodes.
FIG. 1 shows the relation curve of an output power (Po) and a positive driving current (I) for a typical semiconductor laser at various temperatures (T1, T2). The bottom portions of the temperature curves are raised above the x-axis for ease of illustration. As shown in FIG. 1, if a driving current (I) is below a threshold value (Ith), the laser can only emit fluorescence, and the output optical power (Po) has a value of approximately zero mW. Typically, a laser lases when the driving current (I) is above the threshold current (Ith), and the output optical power (Po) linearly rises with the increase of the working current. Therefore, in order to make a laser diode lase, the working current (I) provided to the laser diode must be higher than the threshold current (Ith).
Referring to FIG. 1, the threshold current (Ith) is affected by temperature (T1, T2). Generally, T1 is less than T2. Thus, the higher the temperature (T1, T2), the higher the threshold current (Ith). With the rise of the temperature (T1, T2), the threshold current (Ith) of the laser diode (LD) increases and the slope efficiency decreases.
To compensate for the variation in the threshold value (Ith) and to stabilize the optical power (Po), an automatic power control (APC) circuit 200 is applied, as shown in FIG. 2. FIG. 2 shows a typical circuit embodying an APC 200 that monitors the back illumination of a laser diode (LD) 203, detects the current of a photoelectric diode (PD) 202, and stabilizes the photo-generated current of the photoelectric diode (PD) 202 by adjusting a bias current 201 to the laser diode 203. A modulation current 220 is injected into the circuit 200. The bias current 201 produced by transistor 213 is isolated from the modulation current by an inductor 204.
However, with the rise of temperature, the threshold current (Ith) increases, the slope (S) efficiency of the laser decreases, and subsequently the direct bias current (IBIAS) 201 of the APC circuit 200 increases, which may lead to thermal runaway situations and/or lasers shutting down. Ultimately, this can cause a hard failure of the system and damage to the system's hardware.
This “Discussion of the Background” section is provided for background information only. The statements in this “Discussion of the Background” are not an admission that the subject matter disclosed in this “Discussion of the Background” section constitutes prior art to the present disclosure, and no part of this “Discussion of the Background” section may be used as an admission that any part of this application, including this “Discussion of the Background” section, constitutes prior art to the present disclosure.